The present invention relates to a self-supporting, doped lanthanum manganite air electrode for electrochemical cells, having an excellent thermal expansion match with the solid oxide electrolyte of the cell and also having an appropriate, low, electrical resistivity, and method of making such an electrode.
High temperature, solid oxide electrolyte, electrochemical cells, such as fuel cells, generally contain a calcia-stabilized zirconia support tube, covered successively by a doped lanthanum manganite air electrode, a stabilized zirconia solid electrolyte and a cermet fuel electrode. U.S. Pat. No. 4,414,337 (Ichikawa et al.) taught support tube compositions and methods of making the support tube on which the air electrode and other component fuel cell layers rested. The composition contained by weight: 0.45% to 5.5% organic, water-soluble binder, 1.5% to 4.0% starch, 1.5% to 3.5% cellulose, 0.5% to 2.0% dispersant, 7% to 11% water, and 75% to 89% high temperature refractory material such as calcia-stabilized zirconia, that is (ZrO.sub.2).sub.102 (CaO).sub.1-.chi., aluminum silicate or magnesium silicate. Particle sizes mentioned were as high as 149 micrometers (100 mesh-U.S. Sieve Series), and with 60 wt% to 75 wt% in the 35 micrometer to 53 micrometer range. Here, the starch, cellulose, dispersant and refractory were first mixed, and then added to a solution of the organic binder in water. After de-airing, the composition was formed into a shape and extruded into a tube. One end of the tube was plugged with the same composition which had been previously fired at a temperature higher than the tube, and the other end was fitted with a collar which had been previously fired at a temperature lower than the tube. The whole assembly was then gradually heated from 300.degree. C. to 800.degree. C.
U.S. Pat. No. 4,562,124 (Ruka), relating to air electrodes for high temperature fuel cells, recognized thermal expansion problems between the electrode and electrolyte components. A combination support tube and air electrode having up to 80% density was taught, where cerium was substituted for lanthanum, in the lanthanum manganite air electrode structure. This material had the general chemical formula: EQU La.sub.1-.chi.-.omega. (Ca, Sr or Ba).sub..chi. (Ce).sub..omega. - EQU (Mn or Cr).sub.1-y (Ni, Fe, Co, Ti, Al, In, Sn, Mg, Y, Nb or Ta).sub.y O.sub.3 EQU where: .chi.+.omega.=0.1 to 0.7, y=0 to 0.5, and .omega.=0.05 to 0.25
Cerium was taught as essential and as unique in reducing the coefficient of thermal expansion, although its use appeared to increase the resistivity of the electrode. A variety of materials were mixed, pressed, sintered, and tested vs. La.sub.0.3 Ca.sub.0.5 Ce.sub.0.2 MnO.sub.3, including La.sub.0.35 Ca.sub.0.65 MnO.sub.3, with the composition containing cerium having a much better thermal coefficient match to the (ZrO.sub.2).sub.0.9 (Y.sub.2 O.sub.3).sub.0.1 solid electrolyte composition.
Self-supporting air electrode structures were more particularly described in U.S. Pat. Nos. 4,751,152 (Zymboly) and 4,888,254 (Reichner), where the air electrode structures were of a high bulk type, and a centrally ribbed type which required a plurality of air feed tubes, respectively, both described as made of doped or undoped oxides or mixtures of oxides including but not limited to LaMnO.sub.3, CaMnO.sub.3, LaNiO.sub.3, LaCoO.sub.3, and LaCrO.sub.3, preferably LaMnO.sub.3 doped with Sr.
U.S. Pat. No. 4,276,202 (Schmidberger et al.) also taught LaMnO.sub.3, LaNiO.sub.3 or LaCoO.sub.3 electrodes for fuel cells, but required inclusion of chromium. U.S. Pat. No. 4,174,260 (Schmidberger) related to stacked disc, tubular, compound cells with outer electrodes of La.sub.0.5 Ca.sub.0.5 MnO.sub.3 and inner electrodes of nickel cermet made from nickel particles and stabilized zirconia. The outer electrode was slurry-sprayed over the outer surface of the tubular body and then sintered.
U.S. Pat. No. 4,645,622 (Kock), relating to highly electrically conductive ceramics for fuel cell electrodes, which also have a high resistance against high temperatures and oxidized gases, taught a narrow grouping of La.sub.0.44 to 0.48 Ca.sub.0.42 to 0.50 MnO.sub.3 materials.
What is needed is an air electrode that will be uniquely suitable as a self-supporting air electrode of a thin tubular design, requiring only a single air feed tube, for use in a solid oxide fuel cell, where the fuel cell also contains a stabilized zirconia solid electrolyte and a nickel-zirconia cermet fuel electrode, where the air electrode will have a very close thermal match with the electrolyte and fuel electrode without increasing resistivity. One of the objects of this invention is to provide such an electrode, and a method of making it.